With rapid advance in the calculation speed of the CNC controllers in today's machine tools and in addition to the ever-evolving CAD/CAM software, the use of multiple-axes machining technology is becoming more and more popular and common as well. In conventional multiple-axes machine tools such as five axes milling machines, the swivel spindle head is controlled by remotely mounted servo motors which use timing belts, worm gears and worm wheels, coupled to rotate the spindle head. However, it either suffers the problem of insufficient toque, or it will have a backlash problem resulting from the wear and tear of its worm gears and worm wheels as then are usually made of copper. Some prior arts had addressed the aforesaid problems by designing a swivel spindle head to be driven directly by torque motors, in short, a direct drive swivel spindle head is provided which is featuring by its ability to operate at low rotation speed and high torque. Nevertheless, when a swivel spindle head requires larger torque output, the overall volume of it will increase with the increasing of its torque motor size. Thus, it is in need of a swivel spindle head design for enabling the same to have larger torque output without the trading of the increasing of its volume, and also with lower manufacture cost as well.
There is a gearless, direct drive, multiple axes rotary spindle head for a machine tool, disclosed in U.S. Pat. No. 5,584,621, entitled “Direct drive multiple axes rotary spindle head for milling machine”. However, as the aforesaid spindle head has two torque motors, being arranged symmetrically for directly driving and controlling the rotation, it is required to have the two torque motors to be replaced by some other torque motors with larger torque output when the rotary spindle head requires higher torque, and thus the manufacture cost is increased.
Another such direct drive rotary spindle head is disclosed in U.S. Pat. No. 7,470,095, entitled “Mobile milling head with torque motor drive”. Its reduction gear system is composed of a pair of driving and driven gears while the driven gear is the assembly of two separate plates capable of rotating relative to each other for backlash compensation. However, it is noted that not every tooth of the gears in the reduction gear system can free from backlash by the aforesaid design, and also as the driven gear is composed of two separate plates, the clamping of the two can be loosen when the driven gear is subjected to a comparatively large torque so that its mechanical reliability might not be satisfactory.